In traditional automatic transmissions, hydraulic fluid line pressure is usually maintained at two different pressure levels while in the “Drive” mode. In first and second gears (i.e. the low gears), the pressure level (e.g., about 135 psi) remains constant. When the transmission shifts to third and fourth gears (i.e. the “high gears”), the pressure level typically lowers (e.g., to about 85 psi).
A known automatic transmission having the ability to vary the line pressure according to an optimal running condition pressure is described, for example, in U.S. Pat. No. 6,471,613 to Botosan et al., which is commonly assigned to the assignee of the present disclosure. The known automatic transmission varies the line pressure and operates at an increased pressure level for a low gear situation in addition to operation at a lower pressure level for a high gear situation. Regardless of the gear situation, the known automatic transmission operates in a default mode at a maximum line pressure level. Accordingly, although maximum line pressure is available for low gears to achieve clutch holding torque capacity for control of the clutches, the maximum line pressure capacity is also made available to clutches (e.g., the overdrive clutch) that operate in a higher gear.
It can be desirable to provide an automatic transmission having a variable line pressure arrangement that limits line pressure when operating in the default mode at a maximum line pressure level. Accordingly, the durability of a clutch, such as, for example, the overdrive clutch, operating under a maximum line pressure in a default mode of a variable line pressure automatic transmission may be improved while also eliminating overloading of an overdrive piston, clutch components, and the like to realize optimum performance in addition to avoiding clutch slip.